1. Field of the Invention
This invention relates to a method of and a system for detecting a prospective abnormal shadow in a radiation image and a method of reproducing a radiation image.
2. Description of the Related Art
Conventionally, disease or injury of a patient has been sometimes diagnosed by reading an X-ray film on which a radiation image of the patient is recorded. Recently, there has been developed a system in which a radiation image of a patient is once stored on a stimulable phosphor sheet (or a radiation image conversion panel) by exposing the stimulable phosphor sheet to radiation through the object (patient) to have the stimulable phosphor sheet store radiation energy, and stimulated emission which is emitted from each part of the stimulable phosphor sheet upon exposure to stimulating light in proportion to the radiation energy stored thereon is digitally read, thereby obtaining a digital image signal representing the radiation image stored on the stimulable phosphor sheet, and the digital image signal is reproduced as a visible image on a recording medium such as a photographic film or on an image display system such as those using a CRT. (A radiation image recording and read-out apparatus: computed radiography, see, for instance, Japanese Unexamined Patent Publication Nos. 55(1975)-12429, 56(1976)-11395 and 56(1976)-11397)
In accordance with such a radiation image recording and read-out apparatus, by carrying out image processing on the digital image signal in various ways, the tone properties and the frequency properties of the output visible image can be improved so that the visible image becomes more suitable for reading and diagnose through the visible image can be made more correctly.
Further, there has been developed a system in which the digital image signal is analyzed by a computer to automatically detect an abnormal shadow representing a growth, a micro calcification or the like, whereby a certain detecting level can be ensured irrespective of the skillfulness of the reader. (Prospective abnormal shadow detecting system: see, for instance, Japanese Unexamined Patent Publication Nos. 8(1996)-294479)
In this system, the degree of convergence of density (signal value) gradient vectors is evaluated in a digital image signal of a breast (a mammogram) mainly obtained by breast cancer examination, thereby automatically detecting a prospective growth shadow in the radiation image, or a prospective micro calcification shadow is automatically detected by carrying out morphology operation (e.g., dilation processing, erosion processing, opening processing, and closing processing). The prospective abnormal shadow thus detected is marked with a ROI frame or the like on a visible image reproduced on a display such as of a CRT or a liquid crystal display or on a print. Then the display or the print is used for diagnosis.
Though being substantially high in detecting accuracy, the prospective abnormal shadow detecting system cannot detect an abnormal shadow at one-hundred percent accuracy at present. That is, the conventional prospective abnormal shadow detecting system cannot perfectly remove noise of radiation and/or other noise generated at random and sometimes mistakes such noise for a prospective abnormal shadow. Further it sometimes mistakes normal tissues or normal structures such as the mammary glands for a prospective abnormal shadow. Accordingly, there has been a strong demand for a prospective abnormal shadow detecting system which can detect a prospective abnormal shadow at a higher accuracy.
In view of the foregoing observations and description, the primary object of the present invention is to provide a prospective abnormal shadow detecting method and a prospective abnormal shadow detecting system which can detect a prospective abnormal shadow more accurately.
Another object of the present invention is to provide a radiation image reproducing method which can reproduce a radiation image with a prospective abnormal shadow clearly separated from a normal shadow.
In accordance with the present invention, at least two radiation images of an object are taken in different directions, prospective abnormal shadows are detected in the respective radiation images, and when prospective abnormal shadows detected in the respective radiation images are in substantially the same position on the object, the prospective abnormal shadows detected in the respective radiation images are taken as one final prospective abnormal shadow.
That is, when a tumor or the like which is observed as an abnormal shadow on a radiation image really exists on the object, an abnormal shadow should be detected in the same position in each of radiation images taken in different directions, whereas there is little probability that noise or the like which is generated at random is detected in the same position in each of radiation images taken in different directions. In the case of a normal tissue or a normal structure such as the mammary glands which really exists in the object, there is little probability that such a normal tissue is mistaken for an abnormal shadow in all the radiation images taken in different directions since such a normal tissue is viewed in different shapes according to the direction of taking a radiation image.
This invention has been made on the basis of such a recognition, and in accordance with a first aspect of the present invention, there is provided a prospective abnormal shadow detecting method for detecting a prospective abnormal shadow in a radiation image, characterized by the steps of
detecting a prospective abnormal shadow in each of at least two radiation images of an object taken in different directions,
determining correspondence of positions on the object between the radiation images, and
taking, as a final prospective abnormal shadow, only prospective abnormal shadows which are detected in the respective radiation images and are in substantially the same position on the object.
Preferably a prospective abnormal shadow in each radiation image is detected by a method disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 8(1996)-294479) in which the degree of convergence of density (signal value) gradient vectors is evaluated in a digital image signal, thereby automatically detecting a prospective growth shadow in the radiation image, or a prospective micro calcification shadow is automatically detected by carrying out morphology operation (e.g., dilation processing, erosion processing, opening processing, and closing processing).
The final prospective abnormal shadow is a prospective abnormal shadow which is more probable than that detected in only one of the radiation images.
xe2x80x9cDetermining correspondence of positions on the object between the radiation imagesxe2x80x9d need not be effected three-dimensionally. That is, since the radiation image is only a projection of a three-dimensional object on a two-dimensional plane, positions in the direction of taking the radiation image (in the direction of depth of the radiation image) cannot be obtained from each radiation image. Accordingly, correspondence of positions on the object between the radiation images can be determined only with respect to a direction perpendicular to the planes on which the radiation images are projected. Thus, correspondence of positions on the object between the radiation images may be determined only in one-dimension.
That is, xe2x80x9cthe expression that the prospective abnormal shadows are in the same position on the objectxe2x80x9d need not be interpreted that the positions of the prospective abnormal shadows are three-dimensionally the same as each other but should be interpreted to include the case where the positions of the prospective abnormal shadows are the same as each other only in a predetermined direction but the direction of depth of the radiation image. Further, the expression xe2x80x9cin substantially the same positionxe2x80x9d means that the prospective abnormal shadows need not be strictly in the same position but they are considered to be in the same position when they are close to each other to such an extent that they are adjacent to each other. Especially when the radiation images are not taken simultaneously, the positions on the object are affected by the posture of the object. Accordingly, the range over which the prospective abnormal shadows may be considered to be in substantially the same position may be set to a range which can be empirically considered to be reasonable, e.g., to include a case where the prospective abnormal shadows partly overlap each other or are adjacent to each other.
As the xe2x80x9cat least two radiation images of an object taken in different directionsxe2x80x9d may be, for instance, a cranio-caudal view of the breast taken with the breast compressed laterally and a medio-lateral view or a medio-lateral oblique view taken with the breast compressed vertically, or a front image of the breast and a side image of the breast. In the case of a mammogram where the object is the breast, an abnormal shadow rarely overlaps other shadows and correspondence of positions on the object between the radiation images can be easily determined by the use of the nipple or the wall of the chest. Accordingly, the present invention can be effectively applied to mammograms. Since the mammograms are taken with the breast pressed in different directions according to the direction of taking the mammogram, the state of deformation of the breast can vary according to the direction of taking the mammogram. In such a case, the distance between a given position on the object and a reference position (e.g., the chest wall or the nipple) varies by the state of deformation of the breast and correspondence of positions on the object between the radiation images is broken. In such a case, for example, when the ratio of the distance of a prospective abnormal shadow in one radiation image from one reference position to the distance of the same from another reference position is equal to the ratio of the distance of a prospective abnormal shadow in the other radiation image from one reference position to the distance of the same from another reference position, the prospective abnormal shadows in the respective radiation images may be considered to be in substantially the same position on the object. That is, whether prospective abnormal shadows in two radiation images are in substantially the same position on the object may be determined on the basis of not only the distance but also, for instance, the ratio of distances.
It is preferred that only when prospective abnormal shadows detected in the respective radiation images are in substantially the same position on the object and at the same time are detected to be the abnormal shadows of the same kind, they are taken as the final prospective abnormal shadow. For example, when the prospective abnormal shadow detected in one of the radiation images is a growth shadow and the prospective abnormal shadow detected in the other radiation image is a micro calcification shadow, they are not taken as a final prospective abnormal shadow.
It is preferred that xe2x80x9cthe degree of sureness of detectionxe2x80x9d is determined for the prospective abnormal shadows, and when a prospective abnormal shadow is detected in two or more radiation images and only prospective abnormal shadows which are detected in all the radiation images and are all in substantially the same position on the object are taken as a final prospective abnormal shadow, the degree of sureness of detection for the prospective abnormal shadow is increased.
As disclosed, for instance, in Japanese Patent Application No. 2000-38298, xe2x80x9cthe degree of surenessxe2x80x9d is an index which represents the degree of probability at which the final prospective abnormal shadow really exists. For example, when the final prospective abnormal shadows determined in accordance with the first aspect of the present invention are displayed together with the primary prospective abnormal shadows, the former are given a higher degree of sureness and the latter are given a lower degree of sureness.
In accordance with a second aspect of the present invention, there is provided a radiation image reproducing method characterized in that the final prospective abnormal shadow is reproduced to be distinguishable from the prospective abnormal shadows detected in the respective radiation images (will be referred to as xe2x80x9cthe primary prospective abnormal shadowsxe2x80x9d, hereinbelow).
For example, the final prospective abnormal shadow and/or the primary prospective abnormal shadows may be reproduced as visible images on a display screen of an image display means such as a CRT or a liquid crystal display or may be reproduced as a hard copy recorded on a recording medium such as a photographic film by a printer such as a laser printer.
That the final prospective abnormal shadow is reproduced to be distinguishable from the primary prospective abnormal shadows does not always require that the final prospective abnormal shadow is reproduced together with the primary prospective abnormal shadows. Only the final prospective abnormal shadow may be reproduced so long as the reader can recognize that the reproduced image is of the final prospective abnormal shadow.
It is preferred that the at least two radiation images be reproduced side by side so that the reader can recognize correspondence between the prospective abnormal shadows at a glance.
When the visible images are reproduced on a display screen of an image display means, the layout of the images and the like can be easily and quickly changed on demand.
In accordance with a third aspect of the present invention, there is provided a prospective abnormal shadow detecting system comprising
a primary prospective abnormal shadow detecting means which detects a primary prospective abnormal shadow in each of at least two radiation images of an object taken in different directions,
a position correspondence calculating means which determines correspondence of positions on the object between the radiation images, and
a final prospective abnormal shadow detecting means which takes, as a final prospective abnormal shadow, only prospective abnormal shadows which are detected in the respective radiation images and are in substantially the same position on the object.
It is preferred that the at least two radiation images of an object taken in different directions be a cranio-caudal view of the breast taken with the breast compressed laterally and a medio-lateral view or a medio-lateral oblique view taken with the breast compressed vertically.
It is preferred that only when the primary prospective abnormal shadows detected in the respective radiation images are in substantially the same position on the object and at the same time are detected to be the prospective abnormal shadows of the same kind, the final prospective abnormal shadow detecting means takes them as the final prospective abnormal shadow.
It is further preferred that the prospective abnormal shadow detecting system is further provided with a reproduction control means which controls a reproducing means to reproduce the final prospective abnormal shadow to be distinguishable from the primary prospective abnormal shadows. In this case, it is further preferred that the reproduction control means is further provided with a layout setting means which causes the reproducing means to reproduce the final prospective abnormal and the primary prospective abnormal shadows side by side.
It is preferred that the reproducing means is of a type which reproduces an image on a display screen.
It is preferred that a determining means which determines the degree of sureness of detection for the prospective abnormal shadows be provided, and when the primary prospective abnormal shadow detecting means detects a prospective abnormal shadow in two or more radiation images and the final prospective abnormal shadow detecting means takes as a final prospective abnormal shadow only prospective abnormal shadows which are detected in all the radiation images and are all in substantially the same position on the object, the determining means increases the degree of sureness of detection for the prospective abnormal shadow.